Continuously operating superatmospheric distillation process control and apparatus therefor



Sept. 19, 1967 J. E. RIJNSDORP 3,342,701

CONTINUOUSLY OPERATING SUPERATMOSPHERIC DISTILLATION PROCESS CONTROL ANDAPPARATUS THEREFOR 1O Sheets-Sheet 1 Filed Feb. 24, 1964 I I I IIIIIAIII l I I I I A I I I I I IIIIIJ a VA l 123mm a a 5.55% mage? :22 I

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HIS ATTORNEY J. E. RIJNSDORP Sept. 19, 1967 PROCESS CONTROL ANDAPPARATUS THEREFOR 1O Sheets-Sheet 2 Filed Feb. 24, 1964 2 VA I I I I Rd 5:25: I L a 2 am I. 53523 I I C355 Al WWI 5 33:8 IL! I 4 I I E J 05 sN II E m on A f g a Imvvf v A 3 T Al v 3 I s s m z N m w a a I E a .2 mmmm 3 E12 a: i u w e 2 2N u 225:2 2 g :28 3 u rl I I I I I I I I I I I II I I I I I I I I IAI. I L r I I I I I I I I I I I I I I I I I I I I I II I I I lk r INVENTOR:

JOHANNES E. RIJNSDORP 1 g HIS ATTORNEY 3,342,701 ATION l0 Sheets-Sheet 4I l w I J. E. RIJNSDORP CQNTINUOUSLY OPERATING SUPERATMOSPHERIC DISTILLPROCESS CONTROL AND APPARATUS THEREFOR Sept. 19, 1967 Filed Feb. 24,1964 3 mm WT 15358 I I l $55 INVENTOR;

JOHANNES E. RIJNSQORP W W HIS ATTORNE J. E. RIJNSDORP Sept. 19, 1167 l0Sheets-Sheet 5 Filed Feb. 24, 1964 I1 @I 53853 W 55252 Al I l l I I I Il I .|....rll||illll. lk

INVENTOR:

JOHANNES E. RIJNSDORP BY: W HIS ATTORNE Sept. 19, 1967 ,J. E. RIJNSDORPCONTINUOUSLY OPERATING SUPERATMOSPHERIC DISTILLAT ION PROCESS CONTROLAND APPARATUS THEREFOR 1O Sheets-Sheet 6 Filed Feb. 24, 1964 QUALITYACCUMULATOR INVENTORI INSTRUMENT JOHANNES E. RIJNSDORP i W HIS ATTORNEYFIG,

119 1%fi? W- 9 .1. E. RUME-SDQRP CONTINUOUSLY OPERATING SUPERATMOSPHERICDISTILLATION PROCESS CONTROL AND APPARATUS THEREFOR l0 Sheets-SheetFiled Feb. 24, 1964 FIG.

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JOHANNES E. RIJNSDORP Sept. 19, 1967 J E'RIJNSDQRP 3,342,101

CONTINUOUSLY OPERAiING SUPERATMOSPHERIG DISTILLIATION PROCESS CONTROLAND APPARATUS THEREFOR Flled Feb. 24, 1964 l0 Sheets-Sheet 2% [g a 29 l5l6 CONTROLLER CONTROLLER PM||N. I: 52 3- CONTOLLER 4? PM '1 SELECTOR q RTIIO Y coIIIIIIIILIEII l I I 54 I I I I m. I I I I I I INVENTOR JOHANNESE. RIJNSDORP HIS ATTORNEY Se t. 19, 1967 J. E. RIJNSDORP 3,342,701

CONTINUOUSLY OPERATING SUPERATMOSPHERIC DISTILLIATION PROCESS CONTROLAND APPARATUS THEREFOR Filed Feb. 24, 1964 MIII CONTROLLER f CONTROLLERCONTROLLER FIG.

w I I l I I R 5 L L AU R T N 0 c CONTROLLER 3 FIG.

INVENTORI JOHANNES E. RIJN SDORP BY W W HIS ATTORNE Sept. 19, 1967 J. E.RIJNSDORP 3,342,701

CONTINUUUSLY OPERATING SUPERATMOSPHERIC DISTILLATION PROCESS CONTROL ANDAPPARATUS THEREFOR Filed Feb. 24, 1964 10 Sheets-Sheet H1 INVENTOR:

JOHANNES E. RIJNS DORP HIS ATTORNEY United States Patent 3,342,701CONTINUOUSLY OPERATING SUPERATMOS- PI-IERIC DISTILLATION PROCESS CONTROLAND APPARATUS THEREFOR Johannes E. Rijnsdorp, Amsterdam, Netherlands,assignor to Shell Oil Company, New York, N.Y., a corporation of DelawareFiled Feb. 24, 1964, Ser. No. 346,781 Claims priority, applicationNetherlands, Mar. 19, 1963, 290,361 8 Claims. (Cl. 203--2) Thisinvention relates to a method and apparatus for the control of acontinuously operating distillation process. More particularly, thisinvention relates to a method and apparatus for the control of a processfor the continuous distillation of a variable stream of an intakemixture which is carried out at superatmospheric pressure in a multitraydistillation column having both stripping and rectifying sections;wherein both reflux in the top and reevaporation in the bottom portionof the distillation .colume are used; and in which the rate of flow ofthe intake mixture remains independent of the conditions in thedistillation process, whereby the operating costs of the process areminimized.

In is a general practice in the industry to provide automatic controlfor continuous distillation processes. Suitable correcting conditions inthis regard are, in general, the amount of reflux, the degree of heatapplied in aid of the re-evaporation (to the reboiler), the top productflow (take-off) the bottom product flow (take-oil) the degree of coolingin the condenser, and the degree of preheating of the feed. The qualityof the products is normally kept constant by controlling theseconditions to maintain predetermined temperatures and pressures in thecolumn.

Known control systems have their drawbacks since temperature andpressure are not always good criteria for product quality. Furthermore,operating at the predetermined pressures and temperatures which wouldresult in the required quality of the products, does not always resultin the optimization of the operating costs for the process. With theadvent of suitable stream analyzers for producing a direct indication ofthe quality of the, process products, it now becomes possible to removethe necessity of controlling the column pressure to a predeterminedvalue. Since the variable costs of operating a distillation process aredetermined mainly by the cost of preheating the feed together with thecost of heating in aid of re-evaporation, and since the degree ofre-evaporation is directly related to the pressure in the column, thenif the process is controlled such that the pressure in the column isfree to vary within the permissible operating limits of the column andthe heat which is supplied to the reboiler is controlled in such a Waythat the minimum amount of heat need be transferred from the reboiler tothe condenser to perfect the desired separation, the variable operatingcosts for the process can be reduced to a minimum.

As is often the case in practice, the components to be separated by thedistillation have an increasing relative volatility at decreasingpressure. If now, the process is controlled in such a way that thepressure automatically adjusts itself to the lowest value at which thedistillation process still produces the desired results, thenaccordingly as the pressure becomes lower, less reflux and vapor floware required to obtain a desired separation. This implies that less heatneed be transferred from the reboiler to the condenser to perfect thedesired separation. This is obtained by supplying less heat to thereboiler, resulting in a reduction of the variable costs for theprocess.

The basic control scheme for controlling a distillation 3,342,701Patented Sept. 19, 1967 column in this manner is described in copendingapplication Ser. No. 330,753 filed Dec. 16, 1963, by J. E. Rijnsdorp,entitled, Method and Apparatus for the Control of a ContinuouslyOperating Distillation Process. 5 It should be noted that the economy ofthe distillation process usually benefits less from adjustment of thefeed preheat than from the pressure being allowed to adjust itselffreely. If it is desired to take full advantage of this influence of thepreheating and thereby reduce the variable costs to a minimum, it isnecessary to ascertain whether the preheating of the feed is provided bya relatively expensive or a relatively cheap source of heat incomparison with the source employed for the re-evaporation. It the costof the source of heat for preheating is relatively expensive as comparedwith the cost of the source of heat used for re-evaporation, preheatingshould be used as little as possible or not at all. On the other hand,if comparatively cheap sources of heat are available for the preheating,then the preheating should be utilized to the fullest to minimize theoperating costs for the process. In the control schemes according to thepresent invention, it is assumed that a relatively expensive source ofheat is available for preheating.

The cost of the distillation process can also be reduced by utilizingthe column to its maximum extent by providing for the vapor and liquidloads of the trays in the column to be as high as possible. This resultsin increased production for the column and therefore tends to reduce thecost of distillation per unit of feed. The load of a tray in adistillation column is determined by the liquid flow and vapor flowthrough that tray. When the feed stream increases, the liquid flow andthe vapor flow increases as a result of which the load of the trays alsoincreases. In order that the yield on the capital invested in the plantbe as high as possible, it is therefore desirable that the feed streambe also as high as possible, the limit generally being set by the loadlimit of the trays. According to the invention, the controls for thecolumn required to prevent overloading of the trays are performedautomatically while preserving, however, the automatically set minimumvalues of reflux flow and vapor flow which, at that given feed stream,still afford the desired separation. Thus, it is possible to utilize afeed stream flow which is equal to the maximum permissible value.

It should also be noted that in various operating pressure ranges forthe distillation column, the load on the trays of the column may eitherincrease or decrease with increasing pressure. The control schemesaccording to the invention are for a distillation process operating in apres- 50 sure range where the tray load decreases with increasing vpressure. This is the range where the vapor density has a largeinfluence on the tray load. Control schemes for controlling adistillation process operating in a pressure range where the tray loadincreases with increasing pressure are shown in copending applicationsP-5649 and P-5852, Ser. Nos. 338,765 and 343,545, filed Jan. 20, 1964and Feb. 10, 1964, respectively, by J. E. Rijnsdorp.

Although as pointed out above, the control schemes according to theinvention do not maintain the pressure in the column at a predeterminedvalue but rather allow the pressure in the column to freely adjustitself between the maximum and minimum permissible operating pressurefor the column, sudden or transient pressure variations in the columnmay disturb the smooth and orderly control of the distillation process.Accordingly, it is another feature of this invention to provide controlfor the process whereby sudden pressure variations in the columnpressure are suppressed while still allowing the pressure in the columnto freely adjust itself.

70 It is therefore the primary object of this invention to provide amethod and apparatus for controlling aprocess for the continuousdistillation of a stream of intake mixture wherein the costs ofoperating the process are reduced to a minimum.

It is a further object of this invention to provide a method andapparatus of controlling a distillation process for the continuousdistillation of a variable stream of intake mixture wherein: thepressure in the column is permitted to freely adjust itself within thepermissible limits of the column pressure; the minimum possible quantityof heat is utilized for preheating the feed; and the degree of coolingsupplied by the condenser is adjusted to maintain the load on the traysin the distillation column at values not exceeding the maximumpermissible tray loads, whereby the cost of operating the distillationprocess is minimized when distillation takes place in a pressure rangewhere the tray load decreases with increasing pressure and where thecost of the heating medium used for preheating the feed is relativelyexpensive in comparison with the cost of the heating medium utilized forre-evaporation.

It is still a further object of this invention to provide a method andapparatus for the control of a process for the continuous distillationof an intake mixture containing components having an increasing relativevolatility at decreasing pressure wherein the pressure in the columnautomatically adjusts itself to the lowest value at which the processproduces the desired results; the minimum possible quantity of heat isutilized for preheating the feed; and the degree of cooling supplied bythe condenser is adjusted to maintain the loads on the trays in thedistillation column at values not exceeding the maximum permissible trayloads, whereby the cost of operating the distillation process isminimized when distillation takes place in a pressure range where thetray load decreases with increasing pressure and where the cost of theheating medium used for preheating the feed is relatively expensive incomparison with the cost of the heating medium utilized forre-evaporation.

It is still a further object of this invention to provide a method andapparatus for the control of a process for the continuous distillationof a variable stream of intake mixture wherein the pressure in thecolumn automatically adjusts itself to the lowest value at which theprocess still produces the desired results; sudden pressure variationsin the column are suppressed; the minimum possible quantity of heat isutilized for preheating the feed; and the degree of cooling supplied bythe condenser is adjusted to maintain the loads on the trays in thedistillation column at values not exceeding the maximum permissible trayloads, whereby the cost of operating the distillation process isminimized when distillation takes place in a pressure range where thetray load decreases with increasing pressure and where the cost of theheating medium used for preheating the feed is relatively expensive incomparison with the cost of the heating medium utilized forre-evaporation.

Briefly, according to the invention, the above objects are obtained bycontrolling the amount of reflux, the degree of re-evaporation, the topproduct flow, and the bottom product flow such that the quantity of thetop product formed as represented by the level of the top productaccumulator, and the quantity of the liquid present in the bottom of thecolumn vary between predetermined acceptable limits and the quality ofthe separation obtained satisfies the desired requirements. The supplyof cooling medium to the top product condenser is then maintained at themaximum quantity which will allow the column to operate at a pressurewhich is not below the minimum permissible pressure for the column; thequantity of heat utilized in preheating the feed is maintained at theminimum possible value which will allow the column to function properly;and, the loads of the trays of both the rectifying and strippingsections of the column are prevented from exceeding their respectivemaximum permissible loads.

The pressure in the column is preventedfrorn dropping below the minimumpermissible value determined for the column by measuring the pressure inthe column and reducing the supply of cooling medium to the condenserwhenever the measured pressure drops below the permissible minimumvalue.

The pressure in the column is likewise prevented from rising above themaximum permissible value determined for the column by decreasing thedegree of re-evaporation whenever the measured pressure arises above themaximum permissible value.

The load of the trays of the stripping section of the column isprevented from rising above the maximum permissible load for the traysby increasing the quantity of heat utilized for preheating, and as shownin some of the embodiments, decreasing the degree of condenser cooling,whenever the maximum permissible load of one or more trays of thestripping section is exceeded. The result of increasing the quantity ofheat utilized for preheating is that the flow of liquid from the feedtray to the reboiler, i.e., the flow of liquid in the stripping section,is decreased, resulting in a decrease in the load on the strippingsection trays. As a result of the liquid flow in the stripping sectionbeing decreased, the vapor flow in that section can also be decreasedwhile still preserving the desired separation, resulting in a furtherreduction in the load of the trays in the stripping section.

The load of the trays of the rectifying section of the column isprevented from rising above the maximum permissible load of the trays bydecreasing the degree of condenser cooling whenever the measured load ofone or more rectifying section trays rises above the maximum permissiblevalue. The result of this correcting action is that the vapor density inthe column is increased, resulting in a decrease in the tray load of therectifying section trays.

The objects and advantages of the present invention will be understoodfrom the following description taken with reference to the attacheddrawings wherein:

FIGURE 1 is a diagrammatic representation of the basic control schemeaccording to the invention;

FIGURE 2 is a diagrammatic representation of a modification of the basiccontrol scheme wherein overloading of the stripping section trays iseliminated by simultaneously adjusting the degree of preheating and thedegree of condenser cooling;

FIGURES 3-5 are modifications of the basic control scheme whereinoverloading of the trays of the stripping section of the column iseliminated by sequentially increasing the degree of preheating anddecreasing the degree of condenser cooling;

FIGURE 6 is a diagrammatic representation of a mod ification of aportion of the control schemes shown in FIGURES 1-5 for suppressingsudden pressure variations in the column;

FIGURES 7-10 are diagrammatic representations indicating alternativecontrol schemes for portions of the previously shown control schemes;

FIGURES 11-14 are diagrammatic representations showing variations ofportions of the control schemes shown in the previous figures whereinratio control is used for controlling the quality of the separation;and,

FIGURE 15 is a diagrammatic representation showing a modification ofFIGURE 5.

Referring now to the drawings wherein like reference numerals in each ofthe figures refer to the same structure, FIGURE 1 is a diagrammaticrepresentation of the basic control scheme according to the inventionfor a multitray distillation column 1 which contains both a rectifyingand a stripping section and operates at a pressure above atmospheric.The feed or intake mixture is continually introduced in either a liquidor vapor form into the column 1 to an intermediate stage through apipeline 2; the top product in the vapor phase is discharged through apipeline 3; and the liquid bottom product is discharged through apipeline 4. Coupled to the feed pipeline 2 is a heat exchanger 5 forpreheating the feed. Although only one heat exchanger is shown,obviously a plurality of heat exchangers may be used if required.

Connected to the top product column discharge line 3 is a condenser 7which condenses the vapor in the line 3; the condensate then flows intoan accumulator 8. A part of the condensate from the accumulator 8 isreturned to the top of the column 1 for reflux by a pipeline 9, whilethe liquid top product output is discharged from the accumulator via adischarge pipeline 10.

Re-evaporation for the column 1 is eflected by passing a stream ofliquid from the bottom of the column via a pipeline 11 to a reboiler 12which may, for example, be a heat exchanger, and returning the heatedstream via a pipeline 13 to the column 1. The particular manner in whichre-evaporation is attained, however, forms per se no part of thisinvention ince re-evaporation may be accomplished in other ways, e.g.,with a heating coil in the bottom of the column.

Before discussing the particular control of the process a distinctionmust be made between, inter alia, controlled conditions and correctingconditions. The controlled conditions are those variables in the processwhich are controlled in such a way that in general the differencebetween the measured value of the variable and the set or desired valueof this variable is decreased. This is accomplished by adjusting othervariables or correcting conditions of the process. In the present case,the amount of reflux, the degree of re-evaporation, the rate of topproduct flow, the rate of bottom product flow, the degree of cooling inthe condenser, and the degree of preheating of the feed are suitable ascorrecting conditions. Of these correcting conditions, four, namely, thereflux, the reevaporation, and the top and bottom product flows, areadjusted in dependence on the following four controlled conditions: thequality of the top product, the quality of the bottom product, the topproduct accumulator level, and the bottom level, i.e., the liquid levelin the bottom of of column 1. The remaining two correcting conditions,i.e., the degree of cooling by the condenser and the degree ofpreheating are adjusted respectively to their maximum and minimum valueswhich will permit the process to function properly.

It should be noted that the present process is conceived as adistillation in which the feed rate may vary independ ently of the otherconditions in the distillation process, that is, the distillation columnshould function properly regardless of the quantity of feed supplied.Processes of this type frequently occur in oil refineries where the feeddistilled often originates from another part of the refinery and storagetanks for this feed are not available or cannot be used. The feedstreams can, however, be controlled to a constant or substantiallyconstant value, for example, when storage tanks are available, withoutrequiring any change in the control schemes forming this invention.

As shown in FIGURE 1, the top product flow is adjusted by means of acontrollable valve 14 connected in the pipeline 10. The valve 14 iscontrolled by means of a controller 15 which compares the measured valueof the level in the accumulator 8, as determined by the level gauge 16,with a signal proportional to the desired level in the accumulator,i.e., the set value of the controller. Any differences resulting fromthe comparison produce an output signal from the controller 15 whichtends to adjust the valve 14 in such a way that the difference isdecreased. Thus, if at a given moment, the measured value of theaccumulator level is larger than the set value, the signal supplied fromthe controller 15 tends to open the valve 14 wider. In a similar manner,the bottom level is maintained at the desired value with the aid of alevel gauge 17, a controller 18, and a control valve 19 in the pipeline4.

The amount of reflux for the column 1 is controlled by means of aquality measuring instrument or meter 20 which analyzes the quality ofthe top products flowing through the pipeline 3 and transmits a signalcorresponding to the separation to a controller 21 wherein the measuredand desired quality of the top product are compared. The output signalfrom the controller 21 is coupled to a control valve 22 in the pipeline9. If the measured quality is too low, the output signal from thecontroller 21 tends to open the valve 22 wider and thus increase theamount of reflux; the reverse occurs if the measured quality proves tobe too high.

The degree of re-evaporation necessary to attain the desired separationin the distillation column is controlled by means of a qualityinstrument 23 which measures the quality of the bottom product andtransmits a signal corresponding to the separation obtained to acontroller 24, wherein the measured and desired values of the quality ofthe bottom products are compared. The output signal from the controller24 is coupled via a signal selector 25, which will be more fullyexplained below, to a control valve 26 which is located in the heatingmedium supply line 27 for the reboiler 12. If at a given moment thequality of separation is too low, the valve 26 is opened Wider; therebyincreasing the degree of re-evaporation. The reverse occurs if themeasured quality appears to be too high.

The cooling medium for the condenser 7 is supplied through a pipeline 28having a control valve 29 connected therein. In order to allow thepressure in the column 1 to reach the lowest possible valueat which theprocess will operate satisfactorily, the control valve 29 is normallyopen as wide as possible. With the embodiment chosen in this example,the valve is opened as Wide as possible if the signal supplied to thevalve is at a maximum value.

In order to insure that the pressure within the column does not varyoutside of the permissible pressure range for the column, the pressurein the column is measured by means of a gauge 28 which is coupled to apair of controllers 31 and 32. The controller 31 comparesthe value ofthe measured pressure with the'desired value corresponding to themaximum permissible pressure (P for the column. As long as the pressurein the column does not exceed P the output signal from controller 31 ismaintained at a maximum value. This output signal and that from thecontroller 24 are transmitted-to the signal selector 25 which has anoutput signal equal to the lower value of its two input sig nals. Sinceunder normal operating conditions, the pressure in the column is lowerthan P the output signal from the controller 31 is maintained at itsmaximum value; the result is that the output signal from the controller24 is transmitted to the valve 26 to control the quality of theseparation. Should the pressure in the column become higher than Phowever, the output signal from the controller 31 then becomes smallerin value until it is transmitted by selector 25 to the valve 26 in lieuof the signal from the controller 24; this causes the valve 26 to beclosed an amount suflicient to bring the pressure in the columnapproximately equal to P Controller 32 prepares the value of themeasured pressure with a value corresponding to the minimum permissibleoperating pressure (P )'of the column. As long as the pressure in thecolumnis higher than P the output signal from controller 32 is at amaximum value. The output signal from controller 32 is transmitted via asignal selector 33, Whose function will be more fully explained below,to the control valve 29. Since under normal operating conditions thepressure in the column is higher than P the output signal fromcontroller 32 is maintained at its maximum value and therefore tends tomaintain the valve 29 in its maximum open position. Should the pressurein the column become lower than P the output from controller 32 beginsto decrease, causing the valve 29 to close an amount suflicient to bringthe pressure within the column approximately equal to P It should benoted that for practical reasons it is recommendable to allow for asmall safety margin in the set values of the controllers 31 and 32,i.e., the set values should be set at values corresponding to pressuresrespectively slightly below the maximum and slightly above the minimumpressures for the column.

The degree of cooling supplied by the condenser 7 is also adjusted, ashereinbefore indicated, in response to the load of the trays in therectifying section of the column 1, i.e., the degree of cooling suppliedby the condenser 7 is decreased whenever the maximum tray load for therectifying section trays is exceeded. As shown in the figure, the loadof the trays in the rectifying section is measured with a tray loadgauge 34. The output signal from the gauge 34 is connected to acontroller 35 which compares this signal with a set value correspondingto the maximum permissible rectifying section tray load, Amax. Theoutput signal from controller 35 provides the second input for theselector 33 which, as with selector 25 above, produces an output signalequal to the lowest value of its two input signals. As long as the loadof the trays in the rectifying section does not exceed A ax reek, theoutput signal from controller 35 is maintained at a maximum value whichpreferably is equal to or slightly greater than the maximum outputsignal from controller 32. Assuming that the column is operating in thepermissible pressure operating range and that the load of the rectifyingsection trays has not exceeded A the output signals from controllers 32and 35 are then both at their maximum values; resulting in the selector33 passing a maximum value signal to the valve 29 and therebymaintaining the valve 29 in its maximum open position. Should the loadon the rectifying section trays become larger than Amax mg however, theoutput signal from controller 35 begins to decrease as a result of whichselector 33 passes this output signal to valve 29 to further close thevalve until the measured load has decreased to approximately x recta- 1In order to insure that the maximum load of the trays in the strippingsection of the column 1 is not exceeded, the load of the trays in thissection is measured by means of a second tray load gauge 36. The outputsignal from gauge 36 is connected to a controller 37 which compares themeasured value of the tray load with a set value corresponding to themaximum permissible stripping section tray load, Amax, strip. The outputsignal from the controller 37, which is maintained at a maximum value solong as the load on the trays of the stripping section does not exceedAmax, strip, is connected to a control valve 38 which controls thequantity of heat supplied to the preheater 5. Since in the presentcontrol scheme it is assumed that the cost of heat used for thepreheating is relatively expensive and that therefore the minimumpossible quantity of heat should be supplied to the preheater 5, valve38 is normally maintained in its maximum closed position. This positionis maintained when the signal supplied to the control valve 38 is of amaximum value. As long, therefore, as the load of the trays in thestripping section does not exceed )t Strip, the output signal from thecontroller 37 is at a maximum Referring now to FIGURE 2, there is showna mode of control which is particularly advantageous of the event ofrapid and large variations in the load of the trays. With this mode ofcontrol, the elimination of an overload of the stripping section traysis brought about by two simultaneously occurring correcting actions,i.e., via the feed preheater and via the condenser. As shown in thefigure, the output signal from the minimum pressure controller 32 andthe rectifying section load controller 35 are again connected torespective inputs of the signal selector 33. In this embodiment of theinvention, however, the output signal from signal selector 33 is notconnected directly to the control valve 29, but rather is firstconnected to the input of another signal selector 39. The signalselector 39 compares the output signal from selector 33 with the outputsignal from the stripping section controller 37 and passes the signalhaving the lowest value to the control valve 29 to adjust the quantityof cooling medium supplied to the condenser 7.

Assuming that the maximum loads on the trays of the stripping andrectifying sections of the column are not exceeded, the outputs from thecontrollers 35 and 37 are then at their maximum values and consequentlythe signal selectors 33 and 39 pass the signal from the controller 32 tothe control valve 29 to maintain the valve 29 in the maximum openposition which will permit operation in the permissible pressureoperating range. Should, however, the maximum permissible tray load forthe rectifying section trays be exceeded, then the output signal fromthe controller 35 begins to decrease unit it becomes less than thesignal from the controller 32, at which point it is passed by the signalselector 33 to the signal selector 39. The signal selector 39 thencompares the output signal from controller 35 with the output signalfrom the stripping section controller 37 and, since it is assumed thatthe maximum permissible load for the trays of the stripping section hasnot been exceeded, passes the output signal from controller 35 to thevalve 29, tending to close the valve 29 until the load on the rectifyingsection trays has decreased to approximately A Tech, Similarly, shouldthe load on the stripping section trays be exceeded, then the outputsignal from controller 37 begins to decrease and is passed by theselector 39 to the valve 29; again tending to close the valve 29 untilthe load on the stripping section trays has been reduced toapproximately max. strip.-

The input signal from the controller 37 is also connected to one inputof a controller 40 which compares this signal to the output signal ofsignal selector 33. The controller 40, the output of which is utilizedto adjust the control valve 38 which adjusts the quantity of heatsupplied to the preheater 5, produces a maximum output signal as long asthe output signal from the controller 37 is at a maximum value andthereby maintains the control valve 38 in its maximum closed position aslong as the tray load does not exceed \m Strip, In the value and thecontrol valve 38 is maintained in the closed position. If the loadbecomes larger than i strip, the output signal from controller 37becomes smaller, as a result of which the valve 38 is further openeduntil the load on the trays in the stripping section becomesapproximately equal to k strip.

It should be noted, that in determining the value of the maximumpermissible load for the trays, it is not only necessary to payattention to the dimensions of the trays, but further, the influence ofthe dynamics of the system and the clearances in various parts of thetrays should be considered. This results in the set value of the maximumpermissible tray load of the controllers being slightly lower than themaximum value specified for the trays.

event that the output signal from controller 37 is less than that fromselector 33 and thereby indicating that the maximum tray load for thestripping section has been exceeded, the output signal from controller40 then causes the valve 38 to be further opened until the two signalsreaching controller 40 become equal. This correcting action increasesthe degree of preheating of the feed, resulting in the load on the traysin the stipping section being decreased below the maximum permissibleload. It should be noted that the order of the connections of thecontrollers 32, 35 and 37 to the selectors 33 and 29 can be permuted.

Refering now to FIGURE 3, there is shown a control scheme to alleviatethe overloading of the trays in the stripping section by successivelyadjusting the preheater and the condenser. Such a control scheme is ofparticular importance, when, due to large variations in the load of thetrays in the stripping section, the load of these trays cannot bereduced below the maximum permissible load by merely increasing thedegree of preheating of the feed, i.e., the load of the trays of thestripping section of the column is not reduced below the maximumpermissible load for the trays even when the feed preheater is operatingat maximum capacity. According to this control scheme, the output signalfrom the controller 37 is passed through a pair of signal amplitudediscriminators 41 and 42. The discriminator 41 passes the signal fromthe controller 37 to the control valve 38 so long as the signal fromcontroller 37 is between its maximum value and a predeterminedintermediate value which preferably corresponds to that value of theoutput signal of controller 37 at which the valve 38 is completely open.The discriminator 42 is set to pass a value corresponding to the maximumoutput signal from the controller 37 to a signal selector 43 during thetime that discriminator 41 is passing the output signal from controller37 to the control valve 40. Once the output signal from controller 37has decreased in value below the predetermined intermediate valuementioned above, discriminator 42 then passes the output signal fromcontroller 37 to the selector 43.

To the second input of the signal selector 43, which produces an outputsignal equal to the lesser of its two input signals, is connected theoutput from the rectifying section controller 35; the output signal fromthe signal selector 43 being connected to one input of another signalselector 44 which functions similarly to signal selector 43 and has theoutput of controller 32 connected to its second input. With this controlscheme, as long as the loads on the trays in the rectifying andstripping sections of the column do not exceed their respective maximumpermissible values, the output signals from controllers 35 and 37 are attheir maximum values and hence the valve 29 is under control of thecontroller 32, i.e., the valve 29 is maintained at the maximum openposition which will allow the column to operate at a pressure which isnot less than P Should the maximum permissible load of the rectifyingsection trays be exceeded, then, as in FIGURE 2, the output signal fromcontroller 35 begins to decrease until it is passed by selectors 43 and44 to the valve 29 to further close the valve 29 until the overload ofthe rectifying section trays is alleviated.

With respect to the control for overloading of the trays of thestripping section, as long as the load on these trays does not exceedthe maximum permissible load, the output signal from controller 37 is ata maximum value, and hence is passed via the discriminator 41 to thecontrol valve 38 to maintain the valve in its maximum closed position.Should the maximum load on the trays in the stripping section of thecolumn be exceeded, however, the output signal from controller 37 beginsto decrease and is then passed via the discriminator 41 to the controlvalve 38 to further open this valve and thereby increase the quantity ofheat supplied for preheating. The controller 37 continues to try toreduce the overload on the stripping section trays via the control valve38 until the output signal from controller 37 has reached the presetintermediate value indicating that the control valve 38 is completelyopen. At this time, if the output signal from controller 37 is stilldecreasing, indicating that an overload of the stripping section traysstill exists, the output signal from controller 37 is passed by thediscriminator 42 and the selectors 43 and 44 to the control valve 29 toreduce the amount of cooling medium supplied to the condenser 7, andthereby reduce the overload on the stripping section trays. It should benoted that during the time the output signal from the controller 37 isbeing passed by the discriminator 41, the discriminator 42 is passing amaximum output signal and hence does not aifect the position of controlvalve 29. Although the control scheme has been described for completesequential operation, it is possible by proper selection of the range ofoperation of the discriminators 41 and 42 to institute control via thecontrol valve 29 at any desired point.

Furthermore, it should be noted that the function of the discriminators41 and 42 can be performed by valve positioners interconnecting thevalves 29 and 38. Obviously if such valve positioners are utilized thediscriminators 41 and 42 may be eliminated. As with the embodiment shownin FIGURE 2, the connections of the controllers 32 and 35 and of thediscriminator 42 to the signal selectors 43 and 44 may be permuted.

Referring now to FIGURE 4, there is shown an alternative control schemefor the mode of control represented by FIGURE 3. According to thiscontrol scheme, the signal from the stripping section tray load gauge 36is passed to the controller 37 and to a second controller 45. The setvalue of the controller 37 corresponds to the maximum permissible valueof the load for the tray of the stripping section, Amax, strip, whilethe set value for the controller 45 is slightly lower than that of thecontroller 37. The controller 37 compares the output signal from thegauge 36 with the maximum permissible load for the stripping trays, xstrip, and produces a maximum output signal as long as the measured loaddoes not exceed A strip, The output signal from the controller 45 ismaintained at a maximum value as long as the gauge 36 is not measuringany value larger than A =A, that is, a slightly smaller value than Astrip, With this control scheme, the output signal from the controller45, which controls the positioning of the control valve 38, begins todecrease at an earlier moment in time than the output signal from thecontroller 37 as the load of the trays in the stripping sectionapproaches k thereby beginning to open the valve 38'at an earlier momentthan that at which the valve 29 begins to close. The result of thisaction is that with increasing load, the degree of preheating of thefeed is already increasing just a moment before Amax. mm is reached. Ifthe effect of increasing the degree of preheating of the feed is stillinadequate to maintain the load of the stripping section trays belowAmax. Strip, then the control valve 29 is additionally further closedvia the controller 37. This mode of control performs particularlyfavorably when variations in the load occur rap idly. It should be notedthat as with the previous control schemes, the connection of thecontrollers 32, 35 and 37 to the selectors 43 and 44 can be permuted.

Similar results to that reached with the control scheme shown in FIGURES3 and 4 are reached with the mode of control according to FIGURE 5. Whenutilizing the control scheme of FIGURE 5, tray load gauges 34 and 36 areadjusted such that their output signals are equal when the largestpermissible tray loads in the two sections of the column are reached,i.e., the values of max. strip. and maznrect. are equal max.)- Theoutput signal from the tray load gauge 36 is connected both to thecontroller 45 and to a signal selector 46 which is connected between thetray load gauge 34 and the controller 35. As opposed to the signalselectors described in the aforementioned control schemes, signalselector 46 produces an output signal equal to the maximum value of itstwo input signals and, therefore, transmits a signal corresponding tothe largest tray load of the controller 35. If the value of the trayload in the stripping section measured by the gauge 36 becomes largerthan )t =A, the output signal from the controller 45 causes the valve 38to be further opened. Should the opening of the valve 38 not have thedesired effect of stopping the increase in the load of the trays in thestripping section, then as the stripping section tray load continues toincrease, the output signal from tray load gauge 36 also continues j toincrease until it reaches a value equal to x which is the set value ofcontroller 35. As soon as the input signal to controller 35 from signalselector 46 exceeds A the output signal from controller 35 begins todecrease and close the control valve 29 via the signal selector 44 asexplained above. In a similar manner, if the maximum permissible loadfor the trays in the rectifying section of the column is exceeded, theoutput signal from the tray load gauge 34 causes controller 35 to varythe position of control valve 29 to decrease the load on the trays. Itshould again be noted, that the order of connecting the controllers 32and 35 to the selector 44 can be permuted.

Although with the mode of control according to the invention, the valueof the pressure in the column is not controlled to a predetermined valuebut is merely held within a wide range within which it can vary, it isoften desirable to suppress rapid pressure fluctuations in the column.Preferably this suppression of the rapid pressure fluctuations in thecolumn is attained by means of a controller which is affected both bythe measured value of one of the four-mentioned controlled conditions(particularly the quality of the separation attained) and by the pressureor pressure variations in the column, but which, in the absence of anyrapid pressure fluctuations, supplies a signal which depends only on thecontrolled condition in question. The output signal from the controlleris used to control the degree of re-evaporation.

FIGURE 6 shows a modification of the re-evaporation control of thepreceding figures by which any sudden pressure variations in the columnare suppressed via the valve 26 which controls the supply of heatingmedium to the reboiler 12. In this embodiment, the valve 26 iscontrolled by means of a controller 47 to the first input of which issupplied the signal corresponding to the pressure in the column from thegauge 30. The second input to the controller 47, which is the set pointand is compared with the first input, originates in the controller 24,and is a signal corresponding to the measured value of one of theaforementioned controlled conditions, e.g., as shown in the figure, thequality of the bottom product. Before the output signal from thecontroller 24 is applied to the controller 47, however, the signal ispassed to a selector 48 to which is also applied a set signalcorresponding to P The selector 48 which functions in the same manner asselector 25 in the embodiments shown in the preceding figures preventsthe signal from the controller 24 from attaining a value higher thanthat corresponding to P Although with this control scheme, the output ofcontroller 47 will reflect any changes in pressure in column 1, sincethe set point of controller 47 is continually varying according to theoutput of controller 24, the pressure in the column will not be adjustedto any predetermined value but will still be free to adjust itself,within the predescribed range, to the optimum pressure which will resultin the desired separation.

Under normal operating conditions, i.e., when the pressure neither istoo high nor varies too rapidly, the supply of heating medium to thereboiler 12 is, therefore, controlled as dictated by the quality of theseparation, i.e., the output of controller 24. If, however, the pressurein the column should vary rapidly, pressure gauge 30 and therebycontroller 47 will react sooner than the quality instrument 23 andcontroller 24. As a result, the valve 26 will be rapidly further closedat a sudden rise in pressure and will be rapidly opened at a suddenpressure drop, thereby suppressing the sudden pressure variation. Itshould be noted that in place of the pressure gauge 30 it is possible touse a gauge system having an output proportional to the rate of pressurevariation.

For further illustration of the method and apparatus for controlaccording to the invention, some additional modifications are presentedhere which give partial alternatives to the embodiments alreadypresented; it being understood that these do not exhaust thepossibilities of actually carrying out the process according to theinvention.

In the embodiment shown in FIGURE 7, the top product flow is controlledby means of a flow meter 50 connected to the pipeline and a controller51, while the reflux flow is controlled by means of a flow meter 52connected to the line 9 and a controller 53. The controller 51 comparesthe output signal from the flow meter 50 and the output signal from theaccumulator level controller 15 in a manner which tends to keep the topproduct flow at a constant value. However, when the level of the liquidin the accumulator 8 changes, the output of the controller 15 reflectsthis change and causes the controller 51 to open further the valve 14 inresponse to a rise in the level of the condensate in accumulator 8 andfurther close the valve 14 for a drop in the condensate level in theaccumulator. The controller 53, which attempts to keep the reflux flowconstant, compares the output signal from the flow meter 52 and theoutput signal from the top product quality controller 21. If the qualityof the top product should change, the output signal from the controller21 causes the controller 53 to open further the valve 22 if the qualityof the top product is below specification and further close the valve 22if the quality is above the desired specification.

The control of the accumulator level and of the quality of the topproduct may take place according to the scheme of FIGURE 8. In thisembodiment the quality instrument 2t} and the controller 21 provide theset value for the controller 51 which compares this signal with theindication of the product-pipeline flow meter 50. The valve 14 is thenadjusted by the controller 51 such that the top product meets thequality specification. The reflux flow rate obviously increases as thevalve 14 is further closed and vice versa. This reflux flow is, however,controlled to maintain the liquid level in the accumulator. This isaffected by the controller 53 which operates the valve 22. Thecontroller 53 compares the indication of the flow meter 52 with a signalfrom the level controller 15 which is responsive to the level gauge 16.

The control of the bottom level and of the quality of, for example, thebottom product, may also take place with the aid of flow meters and flowcontrollers. As shown in FIGURE 9, a flow meter 54 measures the flow ofthe heating medium to the reboiler and transmits a signal to acontroller 55 which sets the valve 26 in the pipeline 27. The set valueof the controller 55 originates from the controller 47 which isconnected as was discussed above with regard to FIGURE 5 and similarlyreceives a set point signal indicative of quality. The bottom-levelcontroller 18, which is coupled to the level gauge 17, supplies the setvalue for a controller 56 which receives a signal from the flow meter 57in the bottom product discharge pipeline 4. The controller 56 thenadjusts the valve 19 in the bottom product flow line 4 to the requiredposition.

As shown in FIGURE 10, it is also possible to control the reflux merelyby means of the flow meter 52 in the reflux line 9 which supplies asignal to the controller 53 to operate the reflux valve 22. The setvalue for the controller 53 is supplied by the top product qualitycontroller 21 together with the quality meter 20.

Additionally several control schemes are possible in which a ratiocontroller is used as one of the two quality controllers. An occasion todo so will present itself if, for instance, the quality of theseparation is expressed as the boiling point of that component which isdistributed equally over top pro-duct and bottom product (cut point),and as the sharpness of separation (separation index). In this case aquality meter may be used for the control of the cut point and a ratiocontroller for the separation index.

In FIGURE 11, a ratio controller 60 receives signals from the flowmeters 61 and 54, located respectively in the feed line 2 and in theheating medium line 27. The output signal from ratio controller 60 isthe set value of the controller 47. The controller 60 causes the flow ofheating medium to conform to the feed flow. The flow meter 54 may alsobe placed in the reflux line 9. The controller 60 then controls, viaselector 48 and controller 47, the flow of heating medium to thereboiler 12 in such a way that the controller 21 in FIGURES 15 changesthe reflux until the ratio between feed flow and reflux reaches 13 thedesired value. Additionally, as shown in FIGURE 12, it is possible touse the flow meter 57 in the bottom product flow line 4, in lieu of themeter 61, to act together with the flow meter 54 in the heating mediumline 27 to the reboiler 12. The outputs of these two flow meters areused as the inputs to the ratio controller 60.

FIGURE 13 shows still another form of ratio control as an embodiment ofthe invention. The level controller 15, operated by the level gauge 16,supplies the set values for the controllers 51 and 53 which operate,respectively, the valve 14 in the top product flow line and the valve 22in the reflux line 9. The flow meters 50 and 52 are provided asdescribed above and coupled to the controllers 51 and 53. With thecontrol according to this scheme, the ratio between the top product flowand the reflux flow, as measured by the flow meters 50 and 52respectively, and naturally also the level of the accumulator 8, arekept constant. As appears in FIGURE 14, the two meters 50 and 52 canalso be connected to transmit their signals to the ratio controller 60,which then controls the reflux valve 22 in a similar manner.

A refinement of the control schemes, incorporating a ratio controller,is obtained by supplying the set value of the ratio controller via acomputer, either analog or digital, which makes it possible to takeaccount of the dependence of the desired ratio on the pressure in thecolumn. Thus, for example at a lower pressure the reflux flow or thedegree of re-evaporation can be adjusted at a relatively smaller value.

In some cases, for instance in the control scheme as shown in FIGURE 6,where use is made of a difference between the signals from the pressuregauge 30 and the quality meter 23 in order to suppress rapid pressurevariations via the control valve 26, it is attractive to make use of adamping filter as shown in FIGURE 15. The filter 63 and the selector 48are connected between the pressure gauge 30 and the controller 47 and adirect connection is provided between the pressure gauge 30 and thecontroller 47. The filter '63 does not pass signal variations originatedby rapid pressure variations. These signals pass via the directconnection'to the controller 47, which then, as explained above, adjuststhe valve 26. Only signals originating fromslow pressure variations arepassed by the filter 63. Of course, such signals also follow the directcourse from the pressure gauge 30 to the controller 47.

However, the two signals are fed to the control mech:

anism of'controller 47 in opposition, i.e., in such a way that theycancel each other. As a result, slow pressure variations below P nolonger have any influence on the position of the control valve 26.However, when P is exceeded the selector 48 transmits an overridingsignal to the controller 47 to move the valve 26 toward closed position.The output signal from controller 47 is, in the former case, controlledentirely by the set value of controller 47, which signal is supplied,for instance, by a quality controller or a ratio controller, as has beendiscussed hereinbefore in connection with the prior embodiments.

It is understood that the quality meters mentioned hereinbetore may bedirect quality meters such as ohromatogra-phic analysis apparatus,infrared or ultra-vio1et absorption apparatus, viscometers, refractiveindex meters, or flash point meters. For this purpose one may also usetemperature meters compensated for pressure variations, meters for thedifference in temperature between two trays, meters for the differencein vapor pressure between the product obtained and the desired productor, under certain conditions, controllers for the ratio of two flows. Itis further understood that it is not necessary to connect the qualitymeter directly to the top product or to the bottom product dischargelines which lead from the column or to the accumulator outlet. Sometimesit may be desirable, for instance in view of the accuracy of the qualitymeter, to connect this meter to the mixture on a selected tray in thecolumn having regard to the nature of the distillation process to selecta stage which is effective to yield meaningful data on which control canbe based; nor is it necessary that the locations of the valves 26 and 29be as invariably indicated in the drawings. Thus the valve 26, forinstance, may alternatively be placed in the discharge line of theheating medium from the reboiler. When this medium is steam, the valve26 should be located in the condensate discharge line. The valve 29 maybe similarly located in the discharge line of the cooling medium. Thisvalve may also be located in the line 3 or in the condensed vapor linebetween the condenser 7 and the accumulator 8. In all cases, however,the valve 29 governs the heat transfer from vapor flow to coolingmedium.

Tray loads can be measured with a meter which produces an output signalproportional to the pressure difference across one or more trays; with aheat conductivity meter in the space over the tray or with any otherstandard measuring system for this purpose. Furthermore,

the tray loads can 'be calculated from process conditions utilizing forexample, an analog computer.

It is further understood that the control systems mentioned hereinbeforemay be pneumatic, hydraulic, electric, electronic or mechanical systemsor may constitute a mixture of these types.

I claim as my invention:

1. In a process for the continuous distillation of a stream of intakemixture which is carried out at superatmospheric pressure in a multitraydistillation column having both rectifying and stripping sections,wherein the overhead vapors are condensed in a condenser and collectedin an accumulator; wherein both reflux in the upper portion andre-evaporation in the bottom portion of the column are used; and whereinthe amount of reflux, the degree of re-evaporation, the top product flowand the bottom product flow are controlled such that the top productaccumulator level and the bottom product level in the column areadjusted to predetermined respective levels and the desired quality ofthe separation is attained, the improvement comprising: allowing thepressure in the column to 'freely adjust itself between the maximum andminimum permissible operating pressures for the column; controlling theflow of cooling medium to the condenser, said control being in adirection to supply the maximum amount of cooling to the condenser;measuring the pressure in said column, reducing said flow of coolingmedium only when said measured pressure equals the minimum permissiblecolumn pressure; providing preheating for the stream or intake mixture,controlling the preheating of the intake mixture to the column, saidcontrol being in a direction to supply a minimum quantity of preheat;measuring the load on the trays of the stripping section of said column,increasing the preheat only when said measured load equals the maximumpermissible load on the trays of the stripping section; measuring theload on the trays of the rectifying section of the column, reducing theflow of cooling medium only where said measured load equals the maximumpermissible load on the trays of the rectifying section; whereby thecosts of operating the distillation column are minimized when thedistillation takes place in a pressure range where the tray loaddecreases with increasing pressure and where the cost of the heatingmedium used for preheating the feed is relatively expensive incomparison with the heating medium used for re-evaporation.

2. The process of claim 1 wherein the column pressure is maintainedbetween the maximum and minimum permissible pressures for column by:measuring the pressure in the column; reducing the flow of coolingmedium to the condenser whenever the measured pressure is below a presetvalue corresponding to the minimum permissible pressure; and decreasingthe degree of re-evaporation whenever the measured pressure is above apreset value corresponding the maximum permissible column pressure.

3. The process of claim 2 wherein overloading of the trays of thestripping section is prevented by: increasing the degree of preheatingof said stream of intake mixture whenever the maximum permissible loadon the trays of the stripping section of the column is exceeded.

4. The process of claim 2 wherein overloading of the trays of thestripping section is prevented by: increasing the preheating of saidstream of intake mixture whenever the load of the trays of saidstripping sections exceeds a value slightly less than the maximumpermissible value for said trays; and, reducing the flow of condensercooling medium whenever the maximum permissible load for the trays ofsaid stripping section is exceeded.

5. The process of claim 3 including the step of also reducing the flowof condenser cooling medium to eliminate overloading of the trays of thestripping section of the column.

6. The process of claim 3 including the step of reduc ing the flow ofcondenser cooling medium whenever the maximum permissible load of thetrays of the stripping section is exceeded and the degree of preheatingof the intake mixture for the column has already been increased to themaximum possible value.

7. The process of claim 2 wherein the step of suppressing suddenpressure variations in the column comprises: adjusting the flow ofheating medium utilized for re-evaporation by a control signalproportional to the difference between (1) a signal proportional to themeasured pressure in the column and (2) a signal proportional to thequality of at least one of the top and bottom products, whereby suddenpressure variations in the column are suppressed without the pressureitself being controlled to a constant value.

8. In an apparatus for the continuous distillation of a stream of intakemixture at superatmospheric pressure in a multitray distillation columnhaving both a rectifying and a stripping section, and utilizing bothreflux in the upper portion and re-evaporation in the lower portion ofthe column wherein: a condenser and an accumulator are connected to thedistillation column to collect the top product; a reboiler is connectedto the bottom portion of the distillation column; said improvementcomprising: a top product flow control valve, said top product flowcontrol valve being disposed to control the flow of top product from thecolumn to the condenser; a bottom product flow control valve, saidbottom product flow control valve being disposed to control the flow ofbottom product "from the column; a reflux line, said reflux lineconnecting the accumulator to the column and having a reflux flowcontrol valve disposed therein; a heat flow control valve, said heatflow control valve being disposed to control the quantity of heatingmedium supplied to the reboiler; a coolant flow control valve, saidcoolant fiow control valve being disposed to control the flow of coolingmedium to the condenser; pressure-measuring means disposed to measurethe pressure in the column; a first controller, said first controllerbeing coupled to both said pressure-measuring means and said coolantflow control valve, said first controller being operable to open saidcoolant flow control valve to its maximum position and close saidcoolant flow control valve only when the column pressure equals theminimum permissible column pressure; a second controller, said secondcontroller being coupled to said pressure-measuring means and said heatflow control valve, said second controller being operable to controlsaid heat flow control valve to maintain said column pressure below themaximum permissible column pressure; a preheater, said feed preheaterhaving a third controller to control the quantity of preheat supplied tothe intake mixture; a first tray load gauge disposed to measure the loadon the trays in the stripping section of the column, said first trayload gauge being coupled to said third controller to operate said thirdcontroller to supply the minimum quantity of preheat to the intakemixture required to keep the load on the trays in the stripping sectionbelow the maximum permissible load; a second tray load gauge disposed tomeasure the load on the trays of the rectifying section of the column; afourth controller, said fourth controller being coupled to both saidsecond load tray gauge and said coolant flow control valve to close saidcoolant flow control valve when the maximum permissible load on saidrectifying section trays is exceeded.

References Cited UNITED STATES PATENTS 2,489,949 11/1949 Blair 196-1322,910,521 10/1959 Cobb 203-2 3,034,307 5/1962 Berger 2O2160' X 3,071,5201/1963 Sm-alling 202160 3,249,519 5/ 1966 Cabbage et al 20 32 WILBUR L.BASCOMB, JR., Primary Examiner..

1. IN A PROCESS FOR THE CONTINUOUS DISTILLATION OF A STREAM OF INTAKEMIXTURE WHICH IS CARRIED OUT AT SUPERATMOSPHERIC PRESSURE IN A MULTITRAYDISTILLATION COLUMN HAVING BOTH RECTIFYING AND STRIPPING SECTIONS,WHEREIN THE OVERHEAD VAPORS ARE CONDENSED IN A CONDENSER AND COLLECTEDIN AN ACCUMULATOR; WHERE BOTH REFLUX IN THE UPPER PORTION ANDRE-EVAPORATION IN THE BOTTOM PORTION OF THE COLUMN ARE USED; AND WHERENTHE AMOUNT OF REFLUX, THE DEGREE OF RE-EVAPORATION, THE TOP PRODUCT FLOWAND THE BOTTOM PRODUCT FLOW ARE CONTROLLED SUCH THAT THE TOP PRODUCTACCUMULATOR LEVEL AND THE BOTTOM PRODUCT LEVEL IN THE COLUMN AREADJUSTED TO PREDETERMINED RESPECTIVE LEVELS AND THE DISIRED QUALITY OFTHE SEPARATION IS ATTAINED, THE IMPROVEMENT COMPRISING: ALLOWING THEPRESSURE IN THE COLUM TO FREELY ADJUST ITSELF BETWEEN THE MAXIMUM ANDMINIMUM PERMISSIBLE OPERATING PRESSURES FOR THE COLUMN; COMTROLLING THEFLOW OF COOLING MEDIUM TO THE CONENSER, SAID CONTROL BEING IN ADIRECTION TO SUPPLY THE MAXIMUM AMOUNT OF COOLING TO THE CONDENSER;MEASURING THE PRESSURE IN SAID COLUMN, REDUCING SAID FLOW OF COOLINGMEDIUM ONLY WHEN SAID MEASURED PRESSURE EQUALS THE MINIMUM PERMISSIBLECOLUMN PRESSURE; PROVIDING PREHEATING FOR THE STREAM OR INTAKE MIXTURE,CONTROLLING THE PREHEATING OF THE INTAKE MIXTURE TO THE COLUMN, SAIDCONTROL BEING IN A DIRECTION TO SUPPLY A MINIMUM QUANTITY OF PREHEAT;MEASURING THE LOAD ON THE TRAYS OF THE STRIPPING SECTION OF SAID COLUMN,INCREASING THE PREHEAT ONLY WHEN SAID MEASURED LOAD EQUALS THE MAXIMUMPERMISSIBLE LOAD ON THE TRAYS OF THE STRIPPING SECTION; MEASURING THELOAD ON THE TRAYS OF THE RECTIFYING SECTION OF THE COLUMN, REDUCING THEFLOW OF COOLING MEDIUM ONLY WHERE SAID MEASURED LOAD EQUALS THE MAXIMUMPERMISSIBLE LOAD ON THE TRAYS OF THE RECTIFYING SECTION; WHEREBY THECOSTS OF OPERATING THE DISTILLATION COLUMN ARE MINIMIZED WHEN THEDISTILLATION TAKES PLACE IN A PRESSURE RANGE WHERE THE TRAY LOADDECREASES WITH INCREASING PRESSURE AND WHERE THE COST OF THE HEATINGMEDIUM USED FOR PREHEATING THE FEED IS RELATIVELY EXPENSIVE INCOMPARISON WITH THE HEATING MEDIUM USED FOR RE-EVAPORATION.